Interfacial Intermixing Research Articles

Optimization of ion-beam-deposited Mo/Si multilayers for extreme ultraviolet masks

The aim of our work is to investigate deposition conditions to further optimize the reflectivity of Mo/Si multilayers (MLs) for reflective coatings of extreme ultraviolet mask blanks. Dark-field transmission electron microscopy (TEM) measurements imply interfacial roughness values of 80 to 150 pm. Bright-field TEM images indicate intermixed layer thicknesses of 0.4 to 1.8 nm. We present reflectivity calculations including these two ML imperfections and compare against prior empirical results. Both interfacial roughness and intermixing are predicted to lower the maximum reflectivity. For example, interfacial roughness of 400 pm lowers the maximum reflectivity by ∼2 % . Smoother interfaces with sub-100 pm allow recovery of ∼1.5 % of the reflectivity. Mo/Si intermixing is predicted to lower the maximum reflectivity by up to 6% relative to an ideal ML. Reflectivity could be recovered by ∼3 % by reducing the intermixing depth by only 20% to 30%. We demonstrate ways to reduce roughness or intermixing by ion beam deposition (IBD). Ion bombardment simulations provide estimates of the atom energy distribution arriving at the mask blank surface during Mo and Si deposition and of stopping depths of each atom into the underlying layer. Key IBD parameters to reduce the deposition energy, and hence the intermixing depth, are summarized: beam voltage and deposition pressure. Lower ion beam voltage or higher pressure can together reduce the intermixing depth by at least 20% to 30%. Bright-field TEM measurements of MLs deposited at various deposition conditions confirm the intermixing predictions.

Molecular dynamics simulation of structural and mechanical features of a Polymer-bonded explosive interface under tensile deformation

Polymer bonded explosives (PBX) are kind of particulate-reinforced composite materials in which interface interaction is of great significance to its structural and mechanical features. In this work, effect of temperature and strain rate on the microstructure, mechanical properties and fracture damage mechanism of TATB-F2314 are studied using molecular dynamics simulations. The TATB layers at the TATB-F2314 interface are deformed, leading to a rough and undulate surface that facilitates the formation hydrogen bonds between TATB and F2314. Intermixing phase is characterized for the first time at the TATB-F2314 interface. The interfacial structures and mechanical properties of TATB-F2314 depend strongly on temperature and strain rate. F2314 experiences a ductile-to-brittle transition at its glass transition temperature, which exerts great influence on the structural evolution and failure mechanism of TATB-F2314. The fracture mainly appears on F2314 under a quasi-static or low strain rate tension but transfers to TATB layers near to the interfacial intermixing phase at a high strain rate. Our simulations reveal the effect of temperature and strain rate on the microstructure, mechanical behavior and fracture damage mechanism of TATB-F2314 interface, which is useful for the design, preparation and safe use of PBX.

Corrigendum to “Screening and optimization of processing temperature for Sb2Se3 thin film growth protocol: Interrelation between grain structure, interface intermixing and solar cell performance” [Solar Energy Mater. Solar Cell. 225 (2021) 1–13 111045

Corrigendum to “Screening and optimization of processing temperature for Sb2Se3 thin film growth protocol: Interrelation between grain structure, interface intermixing and solar cell performance” [Solar Energy Mater. Solar Cell. 225 (2021) 1–13 111045

Screening and optimization of processing temperature for Sb2Se3 thin film growth protocol: Interrelation between grain structure, interface intermixing and solar cell performance

Deliverability of controllable deposition processes represents the current state-of-the-art strategy for the development of emerging 1D crystal-structure photovoltaic (PV) materials. For Sb2Se3, a compound with tremendous potential for next-generation cost-efficient thin-film PV, the current reported findings proposed that columnar grain growth protocol is required to promote efficient carrier transport and to achieve highly efficient solar cells. It is still a matter of debate, however, whether the growth of the thin-film absorber should follow the weak contacted nanorod grain structure or the columnar-sintered large grain protocol, in relation with the processing temperature, the under-layer junction partner, and interface intermixing. In this work, close-spaced sublimation processing temperature in conjunction with CdS and TiO2 junction partner layers is systematically investigated towards optimization of Sb2Se3 thin film growth conditions. The desideratum of columnar-sintered large grains protocol is validated and insights into the mechanism of absorber-buffer interface intermixing are provided. Following a systematical technological approach, a solar cell efficiency of 5.3% is demonstrated. Combining classical and advanced measurement techniques, insights into physicochemical processes and device functionality in connection with grain structure, defect density, carrier concentration, and recombination mechanism in Sb2Se3 absorber are provided.

Interfacial intermixing and anti-phase boundaries in GaP/Si(0 0 1) heterostructures

Epitaxial GaP/Si heterostructures grown by molecular beam epitaxy (MBE) and migration-enhanced epitaxy (MEE) have been studied, primarily using aberration-corrected scanning transmission electron microscopy (AC-STEM). Atomically-resolved structure images, which are sensitive to atomic-column intensity revealed the detailed geometry of antiphase boundaries that were both parallel and also inclined to the growth direction. The polar/non-polar GaP/Si interfaces were neither atomically flat laterally nor abrupt vertically. Measurements of intensity profiles using both bright-field and dark-field AC-STEM images, as well as chemically-sensitive g002 dark-field imaging, indicated substantial interfacial intermixing, which increased significantly from ∼1.3 nm (MEE growth at 440 °C) to ∼2.1 nm (MBE growth at 600 °C). The finite interface width will impact theoretical predictions of charge imbalance and strong electric fields across the heterointerface.

Rewritable High-Mobility Electrons in Oxide Heterostructure of Layered Perovskite/Perovskite.

Perovskite oxide SrTiO3 can be electron-doped and exhibits high mobility by introducing oxygen vacancies or dopants such as Nb or La. A reversible after-growth tuning of high mobility carriers in SrTiO3 is highly desired for the applications in high-speed electronic devices. Here, we report the observation of tunable high-mobility electrons in layered perovskite/perovskite (Srn+1TinO3n+1/SrTiO3) heterostructure. By use of Srn+1TinO3n+1 as the oxygen diffusion barrier, the oxygen vacancy concentration near the interface can be reversibly engineered by high-temperature annealing or infrared laser heating. Because of the identical elemental compositions (Sr, Ti, and O) throughout the whole heterostructure, interfacial ionic intermixing is absent, giving rise to an extremely high mobility (exceeding 55000 cm2 V-1 s-1 at 2 K) in this type of oxide heterostructure. This layered perovskite/perovskite heterostructure provides a promising platform for reconfigurable high-speed electronic devices.

Topological surface state manipulation of magnetic damping and surface anisotropy in topological insulator/nonmagnet/CoFe heterostructures

The recent advances of high spin-orbit torque efficiency in ferromagnetic/topological insulator (TI) structures hold great promise for the development of high-performance spintronic devices. However, the roles of spin-momentum-locked Dirac surface states (SSs) and the interfacial magnetic proximity effect (MPE) on dynamic magnetic properties are still under debate. Here, we quantitatively distinguish the manipulation effects of SSs and MPE on magnetic damping and surface anisotropy in TI/nonmagnet/CoFe heterostructures. We found that, in addition to the common spin pumping contribution stemming from SSs, damping enhancement also consists of an obvious MPE contribution to the TI/CoFe material system. Moreover, the increased surface magnetic anisotropy for the CoFe films grown on top of a TI layer is believed to arise mainly from the interfacial atomic intermixing. Our paper sheds light on the effects of SSs and MPE on magnetization dynamics, which offer exciting opportunities for developing TI-based spintronics.

Bi2O2Se:Bi2O5Se High‐K Stack as a 2D Analog of Si:SiO2: A First‐Principles Study

"The interface is the device," a famous phrase coined by Nobel laureate Herbert Kroemer, has highlighted the importance of materials interfaces for the functionalities and performance of electronic devices. Semiconductor:dielectric interfaces play central roles in transistors: one of the main reasons of the success of silicon technology and the bottlenecks of many proposed alternatives, among which are the 2D transistors. Major 2D semiconductors, such as MoS2, WSe2, HfSe2, and ZrSe2, have not yet given rise to workable devices based on native high‐K stacks by thermal oxidation which is highly technologically desired. Recently, this feasibility has been demonstrated in Bi2O2Se, an emerging layered 2D material, with its native high‐K oxide, Bi2O5Se. Understanding the atomic‐scale properties of the Bi2O2Se:Bi2O5Se interface is crucial for the developmental applications of this emerging 2D analogue of Si:SiO2. Herein, the atomic structures and band alignment properties of various Bi2O2Se:Bi2O5Se interfaces with different configurations of the interfacial anion layers are comprehensively studied. The correlation between band offset and interfacial atom intermixing is observed and rationalized. The interfacial anion vacancies are also investigated and it is found that oxygen vacancies are detrimental.

Enhanced all-optical switching and domain wall velocity in annealed synthetic-ferrimagnetic multilayers

All-optical switching (AOS) of the magnetization in synthetic ferrimagnetic Pt/Co/Gd stacks has received considerable interest due to its high potential toward integration with spintronic devices, such as magnetic tunnel junctions (MTJs), to enable ultrafast memory applications. Post-annealing is an essential process in the MTJ fabrication to obtain an optimized tunnel magnetoresistance ratio. However, upon integrating AOS with an MTJ in prospect, the annealing effects on single-pulse AOS and domain wall (DW) dynamics in the Pt/Co/Gd stacks have not been systematically investigated yet. In this study, we experimentally explore the annealing effect on AOS and field-induced DW motion in Pt/Co/Gd stacks. The results show that the threshold fluence (F0) of AOS is reduced significantly as a function of annealing temperature (Ta) ranging from 100 °C to 300 °C. Specifically, a 28% reduction of F0 can be observed upon annealing at 300 °C, which is a critical Ta for MTJ fabrication. Finally, we also demonstrate a significant increase in the DW velocity in the creep regime upon annealing, which is attributed to annealing-induced Co/Gd interface intermixing. Our findings show that the annealed Pt/Co/Gd system facilitates ultrafast and energy-efficient AOS, as well as enhanced DW velocity, which is highly suitable toward opto-spintronic memory applications.

Phonon Bridge Effect in Superlattices of Thermoelectric TiNiSn/HfNiSn With Controlled Interface Intermixing.

The implementation of thermal barriers in thermoelectric materials improves their power conversion rates effectively. For this purpose, material boundaries are utilized and manipulated to affect phonon transmissivity. Specifically, interface intermixing and topography represents a useful but complex parameter for thermal transport modification. This study investigates epitaxial thin film multilayers, so called superlattices (SL), of TiNiSn/HfNiSn, both with pristine and purposefully deteriorated interfaces. High-resolution transmission electron microscopy and X-ray diffractometry are used to characterize their structural properties in detail. A differential 3-method probes their thermal resistivity. The thermal resistivity reaches a maximum for an intermediate interface quality and decreases again for higher boundary layer intermixing. For boundaries with the lowest interface quality, the interface thermal resistance is reduced by 23% compared to a pristine SL. While an uptake of diffuse scattering likely explains the initial deterioration of thermal transport, we propose a phonon bridge interpretation for the lowered thermal resistivity of the interfaces beyond a critical intermixing. In this picture, the locally reduced acoustic contrast of the less defined boundary acts as a mediator that promotes phonon transition.

Size dependence of interfacial intermixing in Fe/Si multilayer

Size effect on interface structure in Fe/Si multilayers and interfacial phase formation as a result of thermal annealing and irradiation with high energy heavy ions has been studied, using a combination of x-ray reflectivity and depth-resolved x-ray diffraction measurements. Study using a single sample with two multilayer structures with different bilayer periods allows us to elucidate the size effects keeping the process parameters identical. In conformity with some earlier studies, in the as-deposited multilayer Fe-on-Si interface is more diffused as compared to Si-on-Fe interface. This interfacial asymmetry is found to diminish with increasing layer thicknesses. Furthermore, β-FeSi2 is formed preferentially at Fe-on-Si interface, which again is more pronounced in the low bilayer period structure as compared to the high bilayer period structure. Thermal annealing results in formation of ε-FeSi, which also preferentially forms at the Fe-on-Si interface. Difference in the structure of the interfaces in terms of concentration gradient, interfacial stresses and disorder may contribute to the observed behavior. In contrast, irradiation with heavy ions results in formation of bcc Fe(Si) alloy and β-FeSi2 phase. Intermixing is significantly stronger in the low bilayer period as compared to the high bilayer period structure. The results can be understood in terms of thermal spike model for irradiation induced modifications.

Magnetization of Fe4N thin films: Suppression of interfacial intermixing using buffer layers

We studied the magnetization (Ms) of 60 nm thick polycrystalline Fe4N thin films deposited using reactive dc magnetron sputtering. The optimum substrate temperature (Ts) to grow an stoichiometric Fe4N phase is about 673 K, in agreement with earlier works. However, at this Ts, we found that significant intermixing is taking place at the substrate-film interface, affecting the Ms of Fe4N film, adversely. We performed secondary ion mass spectroscopy (SIMS) measurements to extract the extent of such intermixing. It was found that Si from the SiO2 substrate get interdiffused into Fe4N film forming an undesired interface extending up to almost half of the film. As a result, the average Ms of Fe4N film get reduced by about 25% as compared to its theoretical value. To prohibit such intermixing, we placed thin layers of Ag, Cu, or CrN as a buffer layer between the substrate and Fe4N film. Such buffer layer suppress the extent of interdiffusion remarkably. From SIMS measurements, we found that among all buffer layers, CrN is most suitable as it reduces the extent of intermixing to a minimum level. Magnetization measurements performed using bulk magnetization and polarized neutron reflectivity (PNR) methods also clearly revealed that the Ms was the highest when CrN buffer layer was used. From these results, it becomes apparent that the intermixing taking place at the substrate-film interface plays a vital role in affecting the Ms of Fe4N thin films. To further suppress such intermixing an attempt was also made to bring down the Ts from 673 to 523 K. Obtained results are presented and discussed in this work.

Local structure and phase change behavior in interfacial intermixing GeTe–Sb2Te3 superlattices

Ge/Sb atomic intermixing in interfacial cationic layers is a common phenomenon for GeTe–Sb2Te3 superlattice (GST-SL) used in memory devices. In this paper, we explored the effect of Ge/Sb intermixing on the phase change behavior of GST-SL upon the heating-quenching procedure. Four interfacial intermixing models of Kooi, Ferro, Petrov and inverted Petrov with different Ge/Sb intermixing ratios (25/75, 50/50 and 75/25) were developed based on the ab initio molecular dynamics. The structural evolution indicated that the Ge/Sb interfacial intermixing could facilitate the structure changes especially for 50/50 Ge/Sb intermixed models. When quenching from 1500 K, more 4-fold Ge-centered octahedrons were produced than tetrahedrons, and the electron localization function further proved that the distorted of Ge(Sb)-centered 6-fold octahedrons were caused by the asymmetrical interactions of Ge–Ge/Sb and Ge–Te. A relatively large Te p orbital contribution in coexisted Ge/Te layer led to a narrower bandgap. In addition, different Ge/Sb atom intermixed ratio which affected the electronic local structure, led to the discrepancy in the initial atom movement of Sb or Ge movement near the gap. The present studies enrich the understanding of Ge/Sb interfacial atomic intermixing effects in GST-SL structural changes.

Enhancement of tunneling electroresistance by interfacial cation intermixing in ferroelectric tunnel junctions

Ferroelectric tunnel junctions (FTJs) have recently aroused extensive attention for applications in information storage. However, the interfacial effect regarding the metallic electrodes on tunneling electroresistance (TER) has still remained unclear in traditional FTJs. Here, we observe the TER enhancement in an all-oxide FTJ, which consists of La0.7Sr0.3MnO3 (LSMO), BaTiO3 (BTO) and Nb:SrTiO3 (NSTO). This FTJ device exhibits a giant ON/OFF current ratio of ~104 at room temperature, which is two orders of magnitude larger than that of Pt/BTO/NSTO FTJ. Using combined microscopic and spectroscopic techniques, we find that Mn-Ti cation intermixing at LSMO/BTO interface is closely associated with the polarization direction of BTO, resulting in the change of the ratio of Mn3+/Mn4+ as well as magnetic exchange interactions. Finally, the interface resistance state is tuned. In addition, the device has the long resistance retention and the high switching reproducibility. Our work emphasizes the crucial role of LSMO electrode and interfacial effect in all-oxide FTJs and provide a feasible way to design the high-performance non-volatile resistive memory devices.

Interfacial intermixing of Ge/Si core-shell nanowires by thermal annealing.

Ge/Si core-shell nanowires (NWs) have huge potential for the realization of high mobility channels in NW field-effect transistors. Thermal annealing is a crucial process for optimizing electrical properties in many applications because it affects the NWs' morphology, crystallinity, dopant activation, and interface intermixing. In this study, we investigated the structural transformation of core-shell NWs at the interface and their thermal stability. The intermixing of Ge and Si atoms at the interface closely depends on, and is enhanced by, the temperature and pressure during annealing, while no intermixing occurred at pressures lower than 6 × 10-6 Pa.

The role of intermixing in all-optical switching of synthetic-ferrimagnetic multilayers

We present a theoretical study of single-pulse all-optical switching (AOS) in synthetic-ferrimagnetic multilayers. Specifically, we investigate the role of interface intermixing in switching Co/Gd bilayers. We model the laser-induced magnetization dynamics in Co/Gd bilayers using the microscopic three-temperature model for layered magnetic systems. Exchange scattering is included, which mediates angular momentum transfer between the magnetic sublattices. In this work, each layer is represented by one atomic monolayer of a GdCo alloy with an arbitrary Co concentration, allowing Co/Gd bilayers with an intermixed interface to be modelled. Our results indicate that within the model intermixing of the Co/Gd interface reduces the threshold fluence for AOS significantly. We show that intermixing does not qualitatively affect the switching mechanism and leads to an increase of the propagation speed of the switching front.

Enhancing domain wall velocity through interface intermixing in W-CoFeB-MgO films with perpendicular anisotropy

We study the influence of He+ irradiation induced interface intermixing on magnetic domain wall (DW) dynamics in W-CoFeB (0.6 nm)-MgO ultrathin films, which exhibit high perpendicular magnetic anisotropy and large Dzyaloshinskii-Moriya interaction (DMI) values. Whereas the pristine films exhibit strong DW pinning, we observe a large increase in the DW velocity in the creep regime upon He+ irradiation, which is attributed to the reduction of pinning centers induced by interface intermixing. Asymmetric in-plane field-driven domain expansion experiments show that the DMI value is slightly reduced upon irradiation, and a direct relationship between DMI and interface anisotropy is demonstrated. Our findings provide insights into the material design and interface control for DW dynamics, as well as for DMI, enabling the development of high-performance spintronic devices based on ultrathin magnetic layers.

THz emission from Co/Pt bilayers with varied roughness, crystal structure, and interface intermixing

Femtosecond laser excitation of a Co/Pt bilayer results in the efficient emission of picosecond THz pulses. Two known mechanisms for generating THz emission are spin-polarized currents through a Co/Pt interface, resulting in helicity-independent electric currents in the Pt layer due to the inverse spin-Hall effect and helicity-dependent electric currents at the Co/Pt interface due to the inverse spin-orbit torque effect. Here we explore how roughness, crystal structure and intermixing at the Co/Pt interface affect the efficiency of the THz emission. In particular, we varied the roughness of the interface, in the range of 0.1-0.4 nm, by tuning the deposition pressure conditions during the fabrication of the Co/Pt bilayers. To control the intermixing at the Co/Pt interface a 1-2 nm thick CoxPt1-x alloy spacer layer was introduced with various compositions of Co and Pt. Finally, the crystal structure of Co was varied from face centered cubic to hexagonal close packed. Our study shows that the roughness of the interface is of crucial importance for the efficiency of helicity-dependent THz emission induced by femtosecond laser pulses. However, it is puzzling that intermixing while strongly enhancing the helicity-independent THz emission had no effect on the helicity-dependent THz emission which is suppressed and similar to the smooth interfaces.

Effects of biaxial strain on interfacial intermixing and local structures in strain engineered GeTe-Sb2Te3 superlattices

Van der Waals (vdW) layered GeTe-Sb2Te3 superlattices (GST-SL) have attracted enormous attentions due to the ultralow power consumption for nonvolatile phase change memory. In this paper, effects of biaxial strain on interfacial intermixing and local structures in strain engineered GST-SL were studied. Highly (0 0 l) textured GST-SL with thickness-varied Sb2Te3 sublayers were fabricated by magnetron sputtering and investigated with multiple analyses on surfaces and interfaces. The appearance of Ge-Sb-Te alloys indicated the interfacial Ge/Sb intermixing in strain engineered GST-SL, which actually were composed of Sb2Te3 quintuple layers, Ge-Sb-Te vdW layers and isolated GeTe bilayers. Grazing incidence x-ray diffraction (GID) and x-ray photoelectron spectroscopy (XPS) results revealed that biaxial strain not only facilitated the interfacial intermixing, but also caused the reconfiguration of Ge-Sb-Te vdW layers. Raman spectra indicated that vertical and in-plane vibrations of GeTe layers were not affected by the interfacial intermixing and the newly-formed Ge-Sb-Te layers had similar vibration characteristics as that of GeTe layers. A modified switching mechanism of GST-SL, relating to both the amorphous-crystalline phase transition of GeTe and Ge-Sb-Te layers incorporated into the Sb2Te3 matrix, was then presented. The present studies shed new light on strain engineering for GST-SL and indicate their potential optoelectronic applications.

Layered Au-Pd-Au nanorod catalysts: Pd-layer thickness effects on catalyst performance

Au/Pd/Au and Au/Pd layered nanorods (NR) with different Pd thickness, down to 15 nm, are electrodeposited in Si-supported porous anodized aluminum thin film templates. These structures are, in contrast to Au-Pd core-shells, mechanically relaxed, so that chemical and electronic interaction effects between Au and Pd are expected to predominate in controlling their performance in the electrooxidation of formic acid. The XRD results are conclusive about the formation of pure metal layers, but a corrugated Au/Pd interface is revealed by scanning electron microscopy. The CV behavior of the different structures shows that the PdO reduction peak is shifted towards more noble values as the Pd thickness decreases. Regarding the electrocatalytic oxidation behavior there is a substantial increase in performance with decreasing Pd-layer thickness. It is further shown that the Au/Pd/Au nanostructures perform better than the Au/Pd bilayer structure for the same Pd layer thickness. Also the long term behavior is improved. Both CV and electrocatalytic behaviors are discussed in terms of interfacial intermixing between Pd and Au at the nanoscale and a higher interface contribution for thinner Pd layers.